The present invention relates to lighting devices and methods. In particular, devices and methods which use one or more light emitting devices and one or more luminescent materials.
In current illumination systems, white light typically covers the Correlated Color Temperature (“CCT”) range of from 2700K to 6500K.
In order make CCT adjustments to current light sources, such as halogen or HID lamps, users are required to use color filters or gels to block unwanted color. In these cases, 20%-50% of light and electrical power is wasted.
Illuminating sources, e.g., lighting devices, may produce “white” light by adding two (or more) distinctly different combinations of colors. While the light emitted by any of these devices will appear white, if the devices are used to illuminate a colored object, which selectively absorbs certain colors, the object might look different when viewed with the two different “white” lights. For this reason, different “white” lights will reproduce colors of objects differently, depending upon the nature of the object.
Color reproduction is typically measured using Color Rendering Index (“CRI”). CRI is a relative measurement of how the color rendition of an illumination system compares to that of a blackbody radiator. The CRI equals 100 if the color coordinates of a set of test colors being illuminated by the illumination system are the same as the coordinates of the same test colors being irradiated by the blackbody radiator. Natural daylight has the highest CRI (of 100), and is considered the aspiring standard for color reproduction ability. In order to quantify color rendering performance, the International Lighting Committee (CIE) originally defined 8 test color samples (TSC) and evaluated the color difference of the TSCs when lit by black body radiator and the light source to be evaluated.
The color difference resulting for each TSC is termed as Ri (where i indicates the number of the TSC, initially set by the CIE from TSC1 to TSC8). CRI is an average value of R1 to R8 (the color difference values for TSC1 to TSC8, respectively) and is considered to be a general indicator of color rendering performance. Later, it was considered that 8 TSCs were not enough and 7 more TSCs (TSC9 to TSC15) were added.
Characteristics of a lighting device can be represented on a 1931 CIE (Commission International de I′Eclairage) Chromaticity Diagram. Those skilled in the art are familiar with this Diagram, which represents the mapping of human color perception as a function of two CIE parameters “x” and “y.” These two parameters create chromaticity coordinates (x, y) which can be plotted in the Diagram. The spectral colors and their associated wavelengths are distributed along the edge of the outlined space, which includes all of the hues perceived by the human eye. The boundary line represents maximum saturation for the spectral colors.
Deviation from a point on the 1931 CIE Chromaticity Diagram can be expressed in either in terms of coordinates, or, in order to indicate as to the extent of the perceived differences in color, in terms of MacAdam ellipses. MacAdam ellipses are, by way of example, a locus of points defined as being ten MacAdam ellipses from a specific hue defined by a particular set of chromaticity coordinates consists of hues which would each be perceived as differing from the specified hue to a common extent (and likewise for loci of points defined as being spaced from a particular hue by other quantities of MacAdam ellipses).
The 1931 CIE Chromaticity Diagram also maps out a blackbody locus or “Planckian” locus, wherein the chromaticity coordinates that lie along the blackbody locus obey Planck's equation: E(λ)=A−5/(e(B/T)−1), where E is the emission intensity, λ is the emission wavelength, T the color temperature of the blackbody and A and B are constants. The human eye perceives white light when chromaticity coordinates lie near or on the Planckian locus. The Diagram was revised in 1976 such that the distance between points on the diagram is approximately proportional to the perceived color difference. Duv represents the distance to the closest point on the Planckian locus on the CIE 1960 (uv) diagram.
Correlated color temperature (“CCT”) is the temperature of the blackbody whose perceived color most resembles that of the light source in question. Chromaticities falling on the black body or Planckian locus are identified by true color temperature while chromaticities near the locus are identified by CCT. In current illumination systems, white light typically covers CCT range of from 2700K to 6500K.
Semiconductor light emitting diodes (LEDs) produce light by exciting electrons across the band gap between a conduction band and a valence band of the semiconductor light-emitting layer. The wavelength of the light generated, when the LED is driven by current, depends upon the semiconductor materials of the light-emitting layers of the LED. Thus, the emission spectrum of any particular LED is concentrated around one wavelength. Because white light is a blend of light of more than one color, it is impossible to produce white light with a single light emitting diode. Prior art devices have achieved reproduction of white light by, for example, employing a pixel made of respective red, green and blue light emitting diodes. Other conventional devices have used a combination of light emitting diodes, e.g., emitting blue light, and a luminescent material that emits, e.g., yellow light, in response to excitation from the light from the LED.
As can be seen in FIG. 1, when white light is produced by combining two colors, the possible perceivable light that can be produced by such a combination graphs as a single line on the 1931 CIE Chromaticity Diagram. In FIG. 1, a hypothetical two color light source can produce light along the dotted line connecting the respective wavelength points of the two light sources along the outer outline edge of the Diagram. In the illustrated hypothetical case, white light could be produced by such a combination of light sources at the point at which the dotted line crosses the Planckian locus, at about 3000K.
On the other hand, when three color light sources are used, the range of possible colors that can be produced is shown on the Diagram as a triangle having vertices corresponding to the wavelengths of the three color sources. For example, FIG. 1 shows a dashed triangle that is typical of a standard RGB (red, green, and blue) device, such as for example in a CRT for a television or computer monitor. Such a device can produce any color within the area of the dashed triangle. As for white light, the RGB display could also produce white at 3000K, as in the two color light, but is not limited to this CCT value. For example, in the illustrated example, white colors ranging from 2700K to 6500K can be produced by the RGB solution. However, as discussed above, while the appearance of the mixed light itself will be similar regardless of the component colors making up the light, the appearance of an object illuminated by various lighting devices, whether two or three color devices, will differ depending upon the component colors of the light source.
While various combinations of phosphors (or “lumiphors”) and light emitting devices have conventionally been utilized to create white light, it has been difficult using solid state light emitters to produce light that will complement natural skin tones, for example in lamps used for makeup mirrors and other similar uses.
U.S. Pat. No. 7,213,940 to Van de Ven et al. (the “'940 patent”) attempts to address the problems relating to the production of pleasing white light by utilizing a device consisting of two types of LEDs and a lumiphor. In particular, Van de Ven proposed a lighting device that includes first and second groups of solid state light emitters, which, when illuminated, emit light having dominant wavelength in ranges of from 430 nm to 485 nm and from 600 nm to 630 nm, respectively, and a first group of lumiphors, which, when excited, emit light having dominant wavelength in the range of from 555 nm to 585 nm.
In the device defined in the '940 patent, the combination of light from the two LED sources and the lumiphor produces a mixture of light having coordinates on the 1931 CIE Chromaticity Diagram that define a polygon shaped area. This area from the '940 patent is shown as area 5 in FIG. 2.
FIG. 2 of the present application shows this area 5 plotted against a Planckian locus. As can be seen from FIG. 2, in the lighting device taught by the '940 patent, the combination of light sources produces a mixture of light having x and y coordinates that defines a point that is within ten MacAdam ellipses of at least one point on the Planckian locus. For example, area 5 as shown on FIG. 2 is defined by five lines, two of which are shown as dashed lines in FIG. 2. As can be seen from FIG. 2, with the two groups of light emitters used in the '940 patent, only white light with a CCT of between 2700K˜4000K can be produced.
There exists a need to provide a lighting device that achieves improved CRI values using solid state light emitters, e.g., LEDs, in order expand their usage in fields such as film, theater, cosmetics, fashion and apparel. There is also a need for a highly efficient white light source using solid state light emitters with improved CRI values and a flexible, wide gamut, i.e., range of accessible colors.